A resistance change type sensor apparatus is known as a sensor apparatus. The resistance change type sensor apparatus is provided with a sensor circuit having a resistance element as a sensor element. The sensor circuit detects change of physical quantity of an object to be measured as change of resistance.
For example, in a diaphragm type pressure sensing apparatus, a semiconductor distortion gauge having a semiconductor element is formed on a surface of a diaphragm. The diaphragm is deformed by a force or a pressure from the outside. The resistance of the semiconductor element changes by piezo-resistance effect which arises when the diaphragm is deformed. The change of the resistance of the semiconductor element is converted to an electrical signal and is outputted.
In such a resistance change type sensor apparatus, current is consumed in the resistance element constantly, since it is necessary to apply current/voltage to the resistance element always.
Further, current being consumed in an amplifier is added to the consumption current, because the electrical signal outputted from the resistance element is very small and needs to be amplified by the amplifier.
Such a resistance change type sensor apparatus is used, in many cases, in a small portable apparatus or an apparatus to be driven by a battery so that the above described current consumptions are not preferable, and thus reduction of power consumption is strongly required for the resistance change type sensor apparatus.
A resistance change type pressure gauge is shown as a sensor apparatus in the Japanese patent publication (Kokai) No. 2003-329531, Pages 6 to 7 and FIG. 6. The operation mode of the resistance change type pressure gauge is changed from a normal mode to a sleep mode automatically, and a power supply is turned off, when operation is not executed during a predetermined time period under use of the pressure gauge.
In the pressure gauge, the operation mode is changed to a sleep mode only when operation signal from the outside is not inputted during a predetermined time period. The change of the operation mode is determined by not being operated by an external signal, and the state of the inside of the pressure gauge is not necessarily reflected.
The pressure gauge continues operation of an ordinary mode, even when the physical quantity of an object to be measured is stable and output of a sense circuit does not change. Thus, power consumption of the pressure gauge is not reduced.
On the other hand, variation of offset or sensitivity exists among sensor apparatuses to detect physical quantity of an object to be measured. In the sensor apparatuses, drift of offset or sensitivity occurs depending on temperature.
The Japanese patent publication (Kokai) No. 2005-37301, Pages 6-7 and FIG. 1 discloses a sensor apparatus which incorporate an adjustment circuit for adjusting sensor output signal. An adjustment circuit adjusts the variation of offset or sensitivity existing among sensor apparatuses.
The sensor apparatus is provided with a temperature sensor and a memory. The memory stores a fixed offset value, a fixed gain adjustment value, an offset to temperature coefficient and a gain to temperature coefficient respectively as adjustment values for each of sensor circuits.
The adjustment circuit adjusts offset and gain of an amplifier which amplifies the sensor output signal by reading the offset value and the gain adjustment value from the memory. The adjustment circuit adjusts the drift being caused by temperature change based on the detection value of the temperature sensor.
However, the sensor apparatus needs a memory to store various adjustment values, an adjustment circuit and, further, a temperature sensor to observe temperature. Therefore, the amount of hardware of the sensor apparatus is large.
The fixed offset value and the fixed gain adjustment value are initial values at the time of sensor shipment. Thus, it is difficult to correctly adjust characteristic change of a sensor element provided in the sensor apparatus which is caused by aging.